1. Field of the Invention
The present invention concerns a non-aqueous solvent secondary battery and, more specifically, a non-aqueous solvent secondary battery having a large initial charge/discharge capacity and excellent charge/discharge characteristics at high temperature.
2. Description of the Related Art
Along with the rapid popularization of portable types of electronic equipment, specifications required for batteries used therein have become more and more stringent such that small and lightweight batteries having high capacity, excellent cyclic characteristics, and performance stability have become in demand. In the field of secondary batteries, the quality of higher energy density that lithium non-aqueous solvent secondary batteries have compared to those of other batteries has become prominent such that the market share of lithium non-aqueous solvent secondary batteries has remarkably increased.
A lithium non-aqueous solvent secondary battery basically consists of a negative electrode formed by coating a film of negative electrode active material mix on both surfaces of a negative electrode current collector comprising an elongate sheet-like copper foil or the like, a positive electrode formed by coating a film of positive electrode active material mix on both surfaces of a positive electrode current collector comprising an elongate sheet-like aluminum foil, and a separator comprising a highly porous polypropylene film or the like disposed between them, in which the negative electrode and the positive electrode are wound into a cylindrical or elliptical shape as to remain insulated from each other by the separator and then the wound electrode body is further pressed into a flattened shape in the case of a square cell, and a negative electrode lead and a positive electrode lead are connected to the negative electrode and the positive electrode at predetermined portions thereof respectively, thereafter being housed in an exterior body of a predetermined shape.
For the non-aqueous solvent used in the non-aqueous solvent secondary battery, since the dielectric constant must be high in order to ionize the electrolyte and ion conductivity must likewise be high under conditions of temperatures varying within a wide range, organic solvent, such as carbonates like propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and diethyl carbonate (DEC), lactones like butyrolactone, as well as ethers, ketones and esters are used and, particularly, mixed solvents of EC and a noncyclic carbonate ester of low viscosity like dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (MEC) have been generally used. However, the use of such materials causes the battery to become swollen when stored at high temperature because of low vapor pressure.
On the other hand, since non-aqueous solvents containing PC or BC have high vapor pressure and high oxidation potential, they are less likely to decompose, thereby effectively evolve less amounts of gas and reduce swelling of the cell, and have excellent low temperature characteristics owing to their low coagulation point.
Further, since negative electrodes use carbonaceous materials such as graphite and amorphous carbon as material for the negative electrode, they cost less but have excellent cycle life, and are therefore generally used. However, the use of non-aqueous solvent electrolyte containing PC or BC diminishes the capacity of the battery during charging due to the rapid decomposition of PC or BC. Particularly, the use of carbonaceous material of increased graphitization degree (natural graphite, artificial graphite) of high capacity gives rise to the problem where PC or BC is more rapidly decomposed such that battery charging does not ensue effectively.
Accordingly, a technique has been devised to suppress decomposition due to reduction of the organic solvent, whereby various compounds are added to a non-aqueous solvent electrolyte for the purpose of controlling the negative electrode surface film (SEI: Solid Electrolyte Interface, hereinafter referred to as “SEI surface film”, and also referred to as a passivated layer) so as not to cause the negative electrode active material to react directly with the organic solvent. For example, Japanese Patent Laid-Open No. H08(1996)-045545 (Claims and columns Nos. [0009] to [0012] and [0023] to [0036]) described below discloses a technique of adding at least one member selected from vinylene carbonate and derivatives thereof to an electrolytic solution of a non-aqueous solvent secondary battery, forming an SEI surface film on the negative electrode active substrate layer thereof with additives before lithium intercalates to the negative electrode at initial charging such that the film acts as a barrier to the intercalation of solvent molecules at the periphery of lithium ions.
Further, for the same purpose, Japanese Patent Laid-Open No. 2001-006729 (Claims and columns Nos. [0006] to [0014]) discloses a technique of adding a vinylethylene carbonate compound as an additive in the non-aqueous solvent electrolyte, while Japanese Patent Laid-Open No. 2001-202991 (Claims and columns Nos. [0006] to [0009]) discloses a technique of adding ketones, and Japanese Patent Laid-Open No. 2003-151623 (Claims and columns Nos. [0008] to [0009], and [0022] to [0031]) discloses a technique of adding at least one of vinylene carbonate, cyclic sulfonic acid or cyclic sulfate ester and cyclic acid anhydride while including vinyl ethylene carbonate. Further, Japanese Patent Laid-Open No. 2000-268859 (Claims and columns Nos. [0007] to [0008]) discloses a technique of adding cyclic acid anhydride, while Japanese Patent Laid-Open No. 2002-352852 (Claims and columns Nos. [0010] to [0013]) discloses a technique of adding a cyclic acid anhydride and a vinyl ethylene carbonate compound, respectively.
Among them, while succinic acid anhydride or a succinic acid anhydride derivative serving as a sort of cyclic acid anhydrides is excellent in suppressing decomposition due to the reduction of PC or BC, the resistance of the SEI film increases, causing the deterioration of the battery's charge/discharge characteristics. Further, where succinic acid anhydride is used, the diglycolic acid anhydride as a sort of cyclic acid anhydrides is preferentially reduced to PC or BC at the negative electrode to form the SEI film, such that the effect of suppressing the decomposition of PC or BC is rather insignificant. In other words, the decomposition of PC or BC cannot be suppressed completely. Further, where succinic acid anhydride or succinic acid anhydride derivative, or diglycolic acid anhydride is added in great amounts, the ionic conductivity of the electrolytic solution is reduced, causing the resistance of the SEI film to increase, leading to the deterioration of the battery's charge/discharge characteristics, as well as the remarkable evolution of gases while charging during storage, thereby causing the battery to swell greatly.
In addition, since the cyclic acid anhydride has poor oxidation resistance at high temperature when charge/discharge cycles are conducted at high temperature, oxidative decomposition proceeds vigorously to evolve great amounts of gases, thereby reducing the effective area of the electrode plate causing its capacity to deteriorate. Further, since the concentration of the cyclic acid anhydride in the cell diminishes due to the oxidative decomposition of the cyclic acid anhydride, the decomposition due to the reduction of the solvent at the negative electrode is not effectively suppressed, accelerating deterioration of its capacity. Particularly, in the battery using carbonaceous material with increased degree of graphitization and high capacity for the negative electrode, deterioration at high temperature occurs remarkably in the case of an electrolyte containing PC or BC.
As a result of various studies made on the mechanism for forming the SEI surface film described above, the present inventors have found that when a cyclic acid anhydride and an aromatic compound having at least one electron donating group are added together in a non-aqueous solvent electrolyte, decomposition due to reduction of the non-aqueous solvent can be prevented effectively, and the impedance of the SEI surface film can be decreased while the charge/discharge characteristics of the battery at high temperature can be further improved without lowering its initial discharge capacity and the amount of gas evolved can be lowered drastically.
While the reason for generating such a result is not yet apparent at present and requires further study, it is believed that the object of the invention can be achieved on the assumption that the aromatic compound containing the electron donating group is oxidized preferentially in relation to the oxidative decomposition of the cyclic acid anhydride during charging, thereby suppressing the evolution of gases caused by oxidative decomposition of the cyclic acid anhydride, and further controlling the reduction of the concentration of the cyclic acid anhydride in the cell, and since negative charges are applied to the aromatic ring because it contains the electron donating group, positive charges generated by oxidation are easily stabilized.
Accordingly, the present invention intends to provide a non-aqueous solvent secondary battery capable of lowering the impedance of the SEI surface film, improving the charge/discharge characteristics of the battery at high temperature without lowering its initial discharge capacity and drastically reducing the amount of gases evolved.